Minerals and gems are crystals. A
crystal can be
made of almost anything. Something is called a
"crystal" if the atoms or molecules that make up
the substance are arranged in a regular, periodic
way. This regular arrangement is called a
lattice. A common example of a lattice is a
carton of eggs. The eggs in an egg carton are
arranged in neat rows and columns, regularly
spaced. Another example of a lattice is the way
that apples are stacked in the produce section of
the grocery store.

Some solid
substances have their atoms and molecules arranged
in lattices, and these solids are called crystals.
This happens at a very small scale, so you can't
actually see the arrangement of the atoms with the
human eye. But it is very much like the examples
of the eggs and the apples, with atoms and
molecules in place of the eggs and apples. A
common crystal is quartz, which is made up of
silicon dioxide molecules arranged in a lattice.
Another common crystal is table salt, which is
made up of sodium and chlorine ions arranged in a
lattice. The silicon that computers chips are
made of is a crystal (a lattice of silicon atoms)
.
And diamonds are actually just a bunch of carbon
atoms arranged in a crystal lattice!

Answer 2:

A crystal is made up of atoms of the same
element
or atoms of different elements [like silica (Si)
or calcium (Ca)], and the atoms have a
regular, repeating arrangement. Crystals are very
ordered, the arrangement of a certain crystal is
always the same. Crystals grow when more layers
of the same elements arranged in the same manner
are added to the outside of the crystal.
Depending on how long the crystal is allowed to
grow and how much space it has to grow in, a
crystal can be very tiny (short time, no space)or
very large (long time, lots of space).

A
mineral is a crystalline solid, meaning that
minerals grow in this ordered manner. Minerals
are naturally occurring and have specific chemical
compositions. For example, quartz is always made
up of only silicon and oxygen (SiO2).

A gem
is a rare mineral or an unusual form of some
mineral that is appreciated for its beauty. Topaz
and garnet are minerals that are also gems.
Emerald is really the mineral beryl, but it is
beryl with a certain impurity that makes it dark
green. What I mean by impurity is that sometimes
a different element will substitute for a usual
element in a mineral's atomic structure. Other
gems are hydrated forms of minerals. For example,
opal is silica and water.